[Python-Dev] Attribute lookup ambiguity

[Pascal Chambon]

Hello

I've already crossed a bunch of articles detailing python's attribute 
lookup semantic (__dict__, descriptors, order of base class 
traversing...), but I have never seen, so far, an explanation of WHICH 
method did waht, exactly.

I assumed that getattr(a, b) was the same as a.__getattribute__(b), and 
that this __getattribute__ method (or the hidden routine replacing it 
when we don't override it in our class) was in charge of doing the whole 
job of traversing the object tree, checking descriptors, binding 
methods, calling __getattr__ on failure etc.

However, the test case below shows that __getattribute__ does NOT call 
__getattr__ on failure. So it seems it's an upper levl machinery, in 
getattr(), which is in chrge of that last action.

Is that on purpose ? Considering that __getattribute__ (at lest, 
object.__getattribute__) does 90% of the hard job, why are these 10% left ?
Can we find somewhere the details of "who must do what" when customizing 
attribute access ?
Shouldn't we inform people about the fact that __getattribute__ isn't 
sufficient in itself to lookup an attribute ?

Thanks for the attention,
regards,
Pascal



=======
INPUT
=======

class A(object):

    def __getattribute__(self, name):
        print "A getattribute", name
        return object.__getattribute__(self, name)

    def __getattr__(self, name):
        print "A getattr", name
        return "hello A"


class B(A):


    def __getattribute__(self, name):
        print "B getattribute", name
        return A.__getattribute__(self, name)

    
    def __getattr__(self, name):
        print "B getattr", name
        return "hello B"
    
    

print A().obj
print "---"
print B().obj
print "---"
print getattr(B(), "obj")
print "-----"
print object.__getattribute__(B(), "obj") # DOES NOT CALL __getattr__() !!!


===========
OUTPUT
===========

A getattribute obj
A getattr obj
hello A
---
B getattribute obj
A getattribute obj
B getattr obj
hello B
---
B getattribute obj
A getattribute obj
B getattr obj
hello B
-----
Traceback (most recent call last):
  File "C:\Users\Pakal\Desktop\test_object_model.py", line 34, in <module>
    print object.__getattribute__(B(), "obj") # DOES NOT CALL 
__getattr__() !!!???
AttributeError: 'B' object has no attribute 'obj'